A typical prior art chopper stabilized amplifier functions by switching between two states. The output voltage at each state is averaged using a low-pass filter (a simple R-C filter). The output of the filter is continuously adjusting towards the filter input and will have a ripple whenever the two amplifier states are different (as is typically the case). In a regular prior art implementation, chopper stabilized amplifiers have a ripple at the amplifier output, even when the amplifier is in a DC state.
The prior art circuit shown in FIG. 1 illustrates the typical approach to chopper stabilization. The circuit of FIG. 1 includes NMOS transistors Mn1 and Mn2; PMOS transistors Mp1, Mp2, and Mp3; differential amplifier AMP; capacitor Cfil; resistors Rfil and R1; current Isense; current source I1; and output Vsense. The amplifier in the circuit has two operating states, as in all chopper designs. Each state has a distinct output voltage at node N1 resulting from non-idealities in the amplifier and will produce a distinct current in transistor MP3. Typically the amplifier output voltages at node N1 will be within a few millivolts of each other and the currents in transistor MP3 will be very similar. In this case, the primary non-ideality of interest is offset between the input devices. In general, the objective of chopper stabilization here is to take the two distinct states (one which represents the inherent offset and the other which will be the opposite but equal magnitude offset) and average them together. The average of the two states should be the ideal output voltage (as if the amplifier had zero offset). Therefore, in a state with a DC input, the amplifier output will be stepping between two voltages and the filtered voltage (at the gate of transistor MP1) is intended to be the average of the two amplifier output voltages at node N1 and will have a ripple resulting from the steps.